1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer or a MFP (Multi-Function Peripheral) which is also called a digital composite apparatus.
2. Description of the Related Art
A tandem type image forming apparatus has been known as an image forming apparatus such as a copier or printer. In a tandem type image forming apparatus, plural photosensitive drums are arranged in parallel. Toner images respectively formed on the photosensitive drums are transferred to a paper sheet, multi-layered on the paper sheet, to obtain a color image. An image forming apparatus of this tandem type has an intermediate transfer belt unit.
Jpn. Pat. Appln. Laid-Open Publication No. 10-293514 describes an example of an image forming apparatus having such an intermediate transfer belt unit.
The image forming apparatus in this example has a unit casing, a conveyor mechanism, and an elevation mechanism. The conveyor mechanism has a conveyor belt suspended between first and second rollers provided in the unit casing. The elevation mechanism moves up and down the conveyor mechanism in relation to the unit casing. When replacing the conveyor belt or the like, the conveyor belt is moved away from a photosensitive drum. The elevation mechanism has a cam and a lever, and rotates the cam in accordance with rotation of the lever, thereby moving up and down the conveyor mechanism.
The conveyor belt and the photosensitive drum need to be apart from each other during maintenance services and in contact with each other during use. When the belt and drum are brought into contact with and moved apart from each other by the cam mechanism, peripheral units and members receive impact in some cases. For example, when plural gears are engaged together through such contact, tooth tips of the gears collide with each other to scratch or leave an impact scar on tooth surfaces of the gears. In this case, driving of the mechanism becomes stiff for each cycle, and causes color drifting or jitters in a formed image.
Jpn. Pat. Appln. Laid-Open Publication No. 2005-91725 describes an image forming apparatus implemented with a measure for relaxing impact as described above. In an example of this apparatus, a distant state of a transfer roller in which the roller is apart from an intermediate transfer belt transits to a contact state. Timing of such transition is adjusted so that an end of the transfer roller in the lengthwise direction is brought into contact with the intermediate transfer belt prior to the other end of the transfer roller.
However, the above measure for relaxing impact requires delicate adjustment for transition timing from a distant state to a contact state, and gives rise to a drawback of complex structure.
The present invention provides an image forming apparatus of a tandem type, which includes a mechanism for moving a transfer unit and a photosensitive drum apart from each other and reduces impact generated when a state of the unit and drum being apart from each other transits to a contact state.